1. Field of the Invention
The invention is directed to a process for etching titanium at a controllable rate.
2. Art Background
Devices such as multichip modules and hybrid integrated circuits are typically fabricated, in part, by depositing one or several layers of metal and/or polymer onto a substrate such as silicon or ceramic. Titanium metal is frequently used in the fabrication of devices to promote adhesion between the substrate and a metal layer, between the substrate and a polymer layer, or between a metal layer and a polymer layer.
The titanium layer is patterned during the fabrication of these devices by forming a patterned polymer mask (denominated a resist mask) over the titanium layer and removing the regions of the titanium metal that are not covered by the resist. The exposed titanium metal is removed by immersing the substrate in an etchant such as aqueous hydrogen fluoride (HF), or by spraying the etchant onto the substrate. HF etches through the native titanium oxide on the titanium surface in an amount of time that is sufficiently short to satisfy processing needs. However, the HF etches the titanium metal underlying the oxide at a much more rapid rate. Thus, an aqueous HF etchant that will etch through a thin layer of oxide (less than 100 .ANG.) in 5 or 10 seconds will etch through a 1000 .ANG.-thick titanium metal layer in less than one second.
The patterns in these devices, i.e., the metallized line dimensions and the spaces between the lines, are becoming increasingly fine. Currently, lines and spaces are typically 150 microns on printed circuit boards, 50 to 100 microns on ceramic substrates for multichip modules, and less than 10 microns on silicon devices. Finer lines and spaces allow greater density of components and active elements.
Aqueous HF etchants etch titanium isotropically. Therefore, regions of titanium that underlie a mask are susceptible to lateral attack from the etchant as the etch proceeds downward. This lateral component of the etch is referred to as undercut. As the linewidth becomes smaller, less undercut is advantageous. Therefore, especially when linewidths are small, it is desirable to terminate the etch as soon as the objectives of the etch are accomplished, i.e., the desired metal has been removed. However, it is extremely difficult to target the precise moment that the desired amount of metal has been removed, especially when the etch rate is rapid. Typically, the substrate is overetched to ensure that the requisite amount of metal is removed from the entire substrate. During the overetch, the etch continues to proceed laterally. Because HF etches titanium so rapidly, it is difficult to terminate the etch at a desired point. Consequently, it is difficult to reduce the degree of undercutting caused by overetch using a predominantly aqueous HF etchant. Therefore, a process that etches titanium in a controllable manner is desired.